Probably structural failure, or pressure of arches, vaults, roof beams. It could also be a land subsidence in contact with foundations.

If there have been earthquakes in the area, the cracks may be a result of horizontal seismic forces.

There is no single way to repair them, depends on the cause that has caused them. Add some pictures.

You can find related answers in "Masonry Structures, Behavior and Design" by Robert Drysdale

in general cracking occurs near the opening in the brick panel. In general shear loading is the prime reason of diagonal cracking in brick panels

Often the reason is in differential settlements, or shear failures due to horizontal loading (earthquakes,...). Retrofitting is often done by injections but first the cause must be removed.

Generally verticals cracks occurs due to uneven settlement of foundation. Horizontal cracks occurs because of mortar failure. Some times cracks occurs at corners of opening, in some cases it occurs due to expansion and contraction of wooden frames or due to shear failure. We can repair this by grouting but we have to remove the cause first.

For more information about cracks see in:

Freeman, T.J, Driscoll, R.M.C, Littlejohn, G.S.(2002): Has your house got cracks, BRE, ICEThomas Tellford, London, p. 154

In case of earthquakes, the repair process of diagonal shear cracks in masonry walls was tested in laboratory. See my paper at 14 IBBMAC, Sydney, 2008.

As said in the question, reasons for cracks in brick masonry can be several. Anyway the direction of the cracks can give indications for their interpretation, therefore a crack pattern survey reported on plan, prospects and sections of the building or structural element can give important indication. In case of seismic actions, the crack direction allows to distinguish between in plane and out of plane forces (e.g. inclined cracks indicate inplane forces). Special attention should be given to vertical cracks in some high rise structures (towers, pillars) subject to heavy weight. In this case also the position of the cracks (at the bottom pf the element or in the middle) can indicate a sort of creep phenomenon which can bring to collapse in a long range (long term behavior of masonry).